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In the heyday of network TV, through the 1970s, a 30 share was considered a benchmark. A series that could not command a 30 share was marked, unofficially, for early cancellation.

As recently as the mid-1980s, the phenomenally popular "Cosby Show" regularly rattled off weekly shares between 50 and 60.

The fragmentation of the television audience, thanks to the spread of cable availability and proliferation of cable channels, has diluted network ratings and shares.

The second milestone also reflects that long-term trend. For the first time, all of the networks fell to single-digit rating averages.

CBS ousted NBC and laid claim to the prime-time ratings crown for the first time since the 1993-94 season.

CBS won with a 9.0 Nielsen rating average and 15 percent share of the audience. NBC barely trailed with an 8.9 rating and 15 share. ABC averaged an 8.1 and 13. Fox finished with a 7.0 and 11, the WB network with a 3.2 and 5, and UPN with a 2.0 and 3.

(A rating represents a percentage of the nation's 99.4 million TV households. A share is a percentage of the households with the sets actually in use. For example, "ER's" 29 share meant that on average, it was seen in 29 percent of the households where sets were turned on.)

CBS also won the season, which began September 23, in a separate category, total viewers, but ranked only fourth in the key demographics, viewers 18 to 34 years old and viewers 18 to 49.

In effect, CBS leads in household ratings because it is strongly favored among viewers 45 and older, while its rivals compete for the younger audience coveted by advertisers.

CBS has been trying to convince advertisers for years that older viewers are no less desirable commercial targets than younger people.

David Poltrack, CBS research chief, said this week that people 45 and older are the most affluent, fastest-growing segment of the population.

Generally speaking, because its viewers are older, CBS is not able to charge as much for commercial time as its competitors. But CBS approaches that liability with an interesting strategy: If you can't join them, beat them. In other words, corral higher household ratings by avoiding the overcrowded battle for younger viewers.

"We're not going the 'Central Park West' and younger route," Les Moonves, president and chief executive officer of CBS Television, said in a telephone interview this week. The reference was to the 1995-96 season, when CBS tried to skew young and took a beating in the ratings. "Central Park West" was one of its failed shows.

Moonves said almost a third of the newly announced fall series on other networks will be teen or "twentysomething" shows all dealing with the lives and problems of young people.

"Our shows deal with adult angst," he said, "which is something I can deal with a lot better."

Moonves said of this season's ratings victory: "This caps a four-year battle from the bottom. When I came to this place a few years ago (in 1995, as president of CBS Entertainment) CBS was the pariah network."

Moonves added that CBS will post a profit this year, something the network failed to do last year.

But even in rising from third place in 1995, CBS has lost ratings as part of network television's downward spiral.

The only network that boosted its ratings this past season was the WB, which posted a 3 percent gain. The WB primarily focuses on teenagers with such shows as "Dawson's Creek," "Felicity" and "Charmed."

Still, "Jesse" was deemed less than a creative triumph, and NBC has promised to make revisions for its sophomore season. It's high ratings are attributed to its placement between "Friends" and "Frasier" on NBC's Thursday night lineup.

In a sense, the more legitimate new hit was "Providence," also on NBC. The Friday night drama about a young doctor who returns to her hometown was jeered by most critics and had no schedule help. Yet it finished the year as a top-20 show.

Although the networks continue to bleed ratings, the news isn't all bad for them.

Business is reportedly brisk, for one thing, in the so-called up-front sales market of advertising time for the year's fourth quarter.

Another possible hopeful sign is the fact that 11 of last fall's new TV series, along with a handful of midseason shows, will return for a second season in the fall. That is a relatively high survival rate. Twenty- six new series from last fall were canceled.

Also, strangely enough, the networks might be able to turn a negative -- their declining ratings -- into a selling point.

The point is that as proliferating channels spread the audience thinly across the TV landscape, the networks deliver the medium's closest approximation of a mass audience.

Collectively, the four major networks and the two "weblets" -- WB and UPN -- claim an average of 62 percent of the prime-time audience.